what are these holes?

[RetFor]

Hi. I picked up a used older trek 3500 to convert to an ebike. I thought it had holes to mount disc brakes in the back, but when I went to put on the adapter, I found that it didn't fit. The holes are only 45 mm apart instead of the is standard 51 mm. Are these an older version before it was standardized to 51 mm? Something else? If they are an older version of disc mounts, anyone know where i can get the proper adapter? My searches haven't turned up anything.

The front fork does have 51 mm holes, although I'm not sure if it is the original fork or not.

Thanks.

[G_M]

Hi. Yeah, those look like the Trek's weird early 2000's disc brake holes. Do you have any Trek dealer nearby? There you should get the adapter relatively cheap or cheaper than on Ebay. Would suggest you to either call them (the closest Trek dealers) or stop by, I believe they can order it straight from Trek.